1. Field of the Invention
This invention relates to a constant-velocity joint through which a driving shaft and a driven shaft are adapted to be coupled together so as to transmit torque from the driving shaft to the driven shaft, and an image forming device.
2. Description of the Related Art
A constant-velocity joint is typically used to transmit torque from a vehicle drive shaft to an axle.
Through such a constant-velocity joint, torque can be transmitted from a driving shaft to a driven shaft such that the driven shaft is rotated at a constant speed even if the angle between the driving shaft and the driven shaft varies. Thus, constant-velocity joints are used in various industrial machines other than motor vehicles, too.
Two types of constant-velocity joints are known: fixed type constant-velocity joints, which only allow the driving and driven shafts to incline relative to each other; and sliding type constant-velocity joints, which allow the driving and driven shafts not only to incline, but also to axially slide relative to each other. A sliding type constant-velocity joint is disclosed in Japanese Patent Publication JP 52-34699 (examined).
The constant-velocity joint disclosed in JP 52-34699 includes an outer ring and an inner member coaxial with the outer ring. A cage is mounted in an annular space defined between the outer ring and the inner member. One of the radially inner and outer walls defining the annular space is formed with three track grooves circumferentially spaced apart from each other at intervals of 120 degrees. Pockets are formed in the cage, each radially aligned with one of the track grooves. A ball is mounted in each of the pockets so as to roll along the track groove. Torque is thus transmitted between the outer ring and the cage through the balls.
Japanese Patent Publication JP 5-341589 (unexamined) discloses an image forming device which includes a photoconductor drum, a motor, and a universal joint through which the shaft of the photoconductor drum and the rotary shaft of the motor are coupled together. The light exposure position and the image transfer position are arranged diametrically opposed to each other on the photoconductor drum so as to cancel any circumferential shrinkage and expansion of images due to fluctuations in the rotational speed of the photoconductor drum at the light exposure position and the image transfer position, so that the transferred images will be substantially free of shrinkage and expansion.
Conventional constant-velocity joints have their outer ring, inner member, cage, and balls all made of a metal or metals. Thus, grease is needed for lubrication of these parts. In order to prevent leakage of grease, it is necessary to close the open end of the outer ring with a boot mounted on the outer ring by tightening a boot band.
Thus, in order to assemble and disassemble the constant-velocity joint, it is necessary to mount and dismount the boot, which is troublesome. Therefore, it is extremely troublesome to replace any damaged or broken parts of the input and output devices between which torque is transmitted through the constant-velocity joint.
While such metal conventional constant-velocity joints are high in strength, they are heavy, need grease lubrication, and tend to produce large operating noise. Their use is therefore limited. For example, they are not suitable for use in office machines, audio devices, medical instruments, food producing machines, household appliances, etc.
In the image forming device disclosed in JP 5-341589, since the universal joint is the non-constant-velocity type, the photoconductor drum cannot be rotated at a constant speed. Although this publication proposes to arrange the exposure position and the transfer position diametrically opposite to each other, it is actually difficult to arrange the above two positions exactly diametrically opposite to each other due to assembling errors and/or inclination of the photoconductor drum. Thus, it will be difficult to completely cancel any shrinkage and expansion of the latent image with the shrinkage and expansion of the toner image. The ultimately obtained image thus tends to be low in quality.
Further, such non-constant-velocity joints do not permit easy disassembling of the input and output devices that are coupled together thorough the joint. In particular, it is troublesome to replace parts of the image forming device such as the photoconductor drum. Maintenance is thus difficult.
An object of the present invention is to provide a constant-velocity joint of which input and output parts can be exchanged easily, which can be easily assembled and disassembled, which needs no lubrication with grease, which is lightweight, compact in size, quiet in operation, and which is less limited in utility.
Another object of the present invention is to provide an image forming device which includes a means for always rotating the photoconductor drum at a constant speed, and of which the maintenance is easy.